lxml.etree offers a lot more functionality, such as XPath, XSLT, Relax NG,
and XML Schema support, which (c)ElementTree does not offer.

etree has a different idea about Python unicode strings than ElementTree.
In most parts of the API, ElementTree uses plain strings and unicode strings
as what they are. This includes Element.text, Element.tail and many other
places. However, the ElementTree parsers assume by default that any string
(str or unicode) contains ASCII data. They raise an exception if
strings do not match the expected encoding.

etree has the same idea about plain strings (str) as ElementTree. For
unicode strings, however, etree assumes throughout the API that they are
Python unicode encoded strings rather than byte data. This includes the
parsers. It is therefore perfectly correct to pass XML unicode data into
the etree parsers in form of Python unicode strings. It is an error, on the
other hand, if unicode strings specify an encoding in their XML declaration,
as this conflicts with the characteristic encoding of Python unicode
strings.

ElementTree allows you to place an Element in two different trees at the
same time. Thus, this:

a=Element('a')b=SubElement(a,'b')c=Element('c')c.append(b)

will result in the following tree a:

<a><b/></a>

and the following tree c:

<c><b/></c>

In lxml, this behavior is different, because lxml is built on top of a tree
that maintains parent relationships for elements (like W3C DOM). This means
an element can only exist in a single tree at the same time. Adding an
element in some tree to another tree will cause this element to be moved.

So, for tree a we will get:

<a></a>

and for tree c we will get:

<c><b/></c>

Unfortunately this is a rather fundamental difference in behavior, which is
hard to change. It won't affect some applications, but if you want to port
code you must unfortunately make sure that it doesn't affect yours.

etree allows navigation to the parent of a node by the getparent()
method and to the siblings by calling getnext() and getprevious().
This is not possible in ElementTree as the underlying tree model does not
have this information.

When trying to set a subelement using __setitem__ that is in fact not an
Element but some other object, etree raises a TypeError, and ElementTree
raises an AssertionError. This also applies to some other places of the
API. In general, etree tries to avoid AssertionErrors in favour of being
more specific about the reason for the exception.

When parsing fails in iterparse(), ElementTree up to version
1.2.x raises a low-level ExpatError instead of a SyntaxError
as the other parsers. Both lxml and ElementTree 1.3 raise a
ParseError for parser errors.

The iterparse() function in lxml is implemented based on the libxml2
parser and tree generator. This means that modifications of the document
root or the ancestors of the current element during parsing can irritate the
parser and even segfault. While this is not a problem in the Python object
structure used by ElementTree, the C tree underlying lxml suffers from it.
The golden rule for iterparse() on lxml therefore is: do not touch
anything that will have to be touched again by the parser later on. See the
lxml parser documentation on this.

ElementTree ignores comments and processing instructions when parsing XML,
while etree will read them in and treat them as Comment or
ProcessingInstruction elements respectively. This is especially visible
where comments are found inside text content, which is then split by the
Comment element.

You can disable this behaviour by passing the boolean remove_comments
and/or remove_pis keyword arguments to the parser you use. For
convenience and to support portable code, you can also use the
etree.ETCompatXMLParser instead of the default etree.XMLParser. It
tries to provide a default setup that is as close to the ElementTree parser
as possible.

The TreeBuilder class of lxml.etree uses a different
signature for the start() method. It accepts an additional
argument nsmap to propagate the namespace declarations of an
element in addition to its own namespace. To assure compatibility
with ElementTree (which does not support this argument), lxml checks
if the method accepts 3 arguments before calling it, and otherwise
drops the namespace mapping. This should work with most existing
ElementTree code, although there may still be conflicting cases.

ElementTree 1.2 has a bug when serializing an empty Comment (no text
argument given) to XML, etree serializes this successfully.

ElementTree adds whitespace around comments on serialization, lxml does
not. This means that a comment text "text" that ElementTree serializes as
"<!-- text -->" will become "<!--text-->" in lxml.

When the string '*' is used as tag filter in the Element.getiterator()
method, ElementTree returns all elements in the tree, including comments and
processing instructions. lxml.etree only returns real Elements, i.e. tree
nodes that have a string tag name. Without a filter, both libraries iterate
over all nodes.

Note that currently only lxml.etree supports passing the Element factory
function as filter to select only Elements. Both libraries support passing
the Comment and ProcessingInstruction factories to select the
respective tree nodes.

ElementTree merges the target of a processing instruction into PI.text,
while lxml.etree puts it into the .target property and leaves it out of
the .text property. The pi.text in ElementTree therefore
correspondents to pi.target + " " + pi.text in lxml.etree.

Because etree is built on top of libxml2, which is namespace prefix aware,
etree preserves namespaces declarations and prefixes while ElementTree tends
to come up with its own prefixes (ns0, ns1, etc). When no namespace prefix
is given, however, etree creates ElementTree style prefixes as well.

etree has a 'prefix' attribute (read-only) on elements giving the Element's
prefix, if this is known, and None otherwise (in case of no namespace at
all, or default namespace).

etree further allows passing an 'nsmap' dictionary to the Element and
SubElement element factories to explicitly map namespace prefixes to
namespace URIs. These will be translated into namespace declarations on
that element. This means that in the probably rare case that you need to
construct an attribute called 'nsmap', you need to be aware that unlike in
ElementTree, you cannot pass it as a keyword argument to the Element and
SubElement factories directly.

ElementTree allows QName objects as attribute values and resolves their
prefix on serialisation (e.g. an attribute value QName("{myns}myname")
becomes "p:myname" if "p" is the namespace prefix of "myns"). lxml.etree
also allows you to set attribute values from QName instances (and also .text
values), but it resolves their prefix immediately and stores the plain text
value. So, if prefixes are modified later on, e.g. by moving a subtree to a
different tree (which reassigns the prefix mappings), the text values will
not be updated and you might end up with an undefined prefix.

etree elements can be copied using copy.deepcopy() and copy.copy(),
just like ElementTree's. However, copy.copy() does not create a
shallow copy where elements are shared between trees, as this makes no sense
in the context of libxml2 trees. Note that lxml can deep-copy trees
considerably faster than ElementTree, so a deep copy might still be fast
enough to replace a shallow copy in your case.